When calling curl_exec twice with RETURNTRANSFER option there's a (reproductible) core dump. Here's a test case :

$ch=curl_init();
curl_setopt($ch,CURLOPT_RETURNTRANSFER,1);
curl_setopt($ch,CURLOPT_URL,'http://www.yahoo.com');
curl_exec($ch);
echo('this text is displayed');
curl_exec($ch);
echo('this text is not displayed (core dump instead)');

The problem shows up in 4.1.0RC1 AND 4.1.0RC2 (but not 4.0.6). If you remove the RETURNTRANSFER option then it works fine. I'm affraid I don't have the knowledge to compile a debug version and give more infos.